1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for controlling wireless voice quality, and specifically to location based voice coder selection by the subscriber.
2. Background and Objects of the Present Invention
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications ever. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. A standardization group, Global System for Mobile Communication (GSM), was established in 1982 to formulate the specifications for mobile cellular radio systems.
With reference now to FIG. 1 of the drawings, there is illustrated a Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Services Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18. Each Location Area 12 is divided into a number of cells 22. Mobile Station (MS) 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless. The MS may also include a Subscriber Identity Module (SIM) 13, or other memory, which provides storage of subscriber related information, such as a subscriber authentication key, temporary network data, and service related data (e.g. language preference).
The MSC 14 is in communication with at least one Base Station Controller (BSC) 23, which, in turn, is in contact with at least one Base Transceiver Station (BTS) 24. The BTS is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible. It should be understood that the BSC 23 may be connected to several base transceiver stations 24, and may be implemented as a stand-alone node or integrated with the MSC 14. In either event, the BSC 23 and BTS 24 components, as a whole, are generally referred to as a Base Station System (BSS) 25.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the Mobile Stations 20 currently located within the MSC/VLR area 12. If a MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that Mobile Station 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
The digital GSM system uses Time Division Multiple Access (TDMA) to handle radio traffic in each cell 22. TDMA divides each frequency (carrier) into eight time slots (physical channels). Logical channels are then mapped onto these physical channels. Examples of logical channels include traffic (speech) channels (TCH) and Control Channels (CCH). However, due to bandwidth limitations, only a limited amount of coded speech can be sent. Therefore, voice coders are used to lower the bit rate for each speech channel.
Furthermore, it is not possible to transmit the speech using conventional encoding methods because the bit rate must be significantly lowered to account for the bandwidth limitations. Therefore, information about the speech, such as filter parameters and the excitation sequence, must be transmitted in addition to the speech encoded at a lower bit rate.
Voice coders perform this operation by filtering the speech signal, calculating the filter parameters, and estimating the excitation in the speech, e.g., toning-toneless noise and the frequency of the vocal cords. What is then transmitted over the air is the filter parameters and the excitation sequence information. These filter parameters and excitation information are then used to recreate understandable speech of good quality, as is understood in the art.
If a relatively high bit-rate can be accepted, good quality can be achieved with simple voice coders of the waveform type. The 64 kbit/s Pulse Code Modulation (PCM) coder is one example. Recent advances in waveform coders have reduced the bit-rate to as low as 16 kbit/s, with future advances potentially lowering it even further.
Another type of speech coder is the vocoder. Vocoders use low bit-rates, and typically produce speech that has a synthetic and metallic tone, due to the reduced sampling rate. This makes it difficult to ascertain the speaker's identity, although the actual words are easily understood.
Hybrid coders, or "soft" vocoders, are more robust than true vocoders. The hybrid approach is a kind of coding that feeds a carefully optimized excitation signal to a linear predictive filter. Hybrid coders adopt many of the efficiencies of traditional vocoding, while following the subtle properties of the speech waveform. The approach uses high quality waveform coding principles to optimize the excitation signal, instead of using the rigid two-state excitation of vocoding.
Low bit-rate coding of voice, whether accomplished by waveform coders, vocoders, or hybrid coders, is critical for accommodating more users on channels that have inherent limitations of bandwidth or power, such as cellular radio or satellite links. As the bit-rate is reduced, quality naturally drops off, unless the complexity of the coding scheme is increased. However, complexity raises the cost, and in many cases, increases the processing delay.
Network operators typically provide incentives, e.g., reduced rates, to subscribers who purchase low bit-rate, e.g., half-rate, channel voice coder usage in order to support more speech channels per time slot. However, in many situations, subscribers who have selected, for example, a low bit-rate, e.g., half-rate, voice coder would prefer to have a high bit-rate, e.g., full-rate, voice coder. For example, if a subscriber is at home or in the office, voice quality may be of more importance than if the subscriber is in the car or out of town. Unfortunately, there is no known method of improving the voice quality based on the location of the subscriber.
It is therefore one object of the invention to allow subscribers to select a voice coder, such as full-rate or half-rate, based on the location.
It is a further object of the invention to store the location-based voice coder selection information within the Mobile Station itself.
It is still a further object of the invention to store the location-based voice coder selection information within the cellular network.